Our immediate goals are: 1) To determine the molecular basis for the decreases in hepatic mitochondrial content and cytochrome oxidase activity in aged Fisher 344 rats. 2) Determination of mitochondrial three-dimensional structure as a function of aging using a digitizer interfaced to a minicomputer. Our primary model will be Fisher 344 rats from the aging colony of NIA. For comparative purposes, other animals will be evaluated i.e., the Sprague-Dawley rat (continues to grow beyond sexual maturity), and partially hepatectomized rats (to compare mitochondria from newly synthesized liver cells in old animals with those of old liver cells.